Not so. Brake rotors are not hardened. They are usually made of ferritic gray iron.

The melting point of a material is only important for the foundry, not for rotor performance. Pure iron has a much higher melting point than carbon steel or cast iron, yet it is soft and has little strength, so melting point has nothing to do with metal performance.

It's rather hard to screw up cast iron metallurgy as it relates to rotors. The chemistry control for gray iron industry is not tightly controlled, nor should it be tightly controlled. Gray iron is produced to specified grades of minimum strength and ductility limits. Rotors are the least demanding of gray iron material properties. The only stringent requirement I remember is that max carbon flake size should be 0.01" MIN so that brake rotors won't be noisy. Most applications have a MAX flake size requirement for strength. It's usually poor foundry practices that result in a bad product, i.e., porosity, improper shakeout, and bad sand control. I've worked in the foundry business and qualified product for the automotive industry.

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95 E320 Cabriolet, 169K